As a glass substrate for various kinds of displays, particularly, a glass substrate in which a metal or oxide thin film is formed on the surface thereof, it is preferable to use an alkali-free glass substrate which does not substantially contain alkali metal ions since alkali metal ions are diffused into the thin film to degrade film characteristics when an alkali metal oxide is contained.
The alkali-free glass substrate to be used for the above-described purpose is obtained by: heating and melting raw materials, which has been mixed in a predetermined mixing ratio, in a melting furnace to achieve vitrification; clarifying the molten glass; subsequently forming a glass ribbon having a predetermined thickness by a float process or a fusion process; and cutting the glass ribbon into a predetermined shape.
On the surface of a glass sheet obtained by forming by the float process or fusion process, minute irregularity and undulation such as distortion and corrugation are present.
In case of the distortion, thickness subtly varies at the parts where the minute irregularity and undulation are present and the distortion is mainly attributable to local heterogeneity of the composition of the molten glass. The distortion becomes noticeable through enlargement of a difference between viscosity characteristics based on the heterogeneous composition of the molten glass as the glass ribbon is thinly drawn in a planar manner. A basic countermeasure against the distortion is to improve the homogeneity of the molten glass.
On the other hand, in case of the corrugation, although the thickness is substantially constant, the minute irregularity and undulation wave at a fine pitch in a width direction of the glass ribbon and it is considered that the corrugation is generated in the process of thinly drawing the glass ribbon in a planar manner in the width direction and in the flow direction mainly in the forming process. A basic countermeasure against the corrugation is to adjust a tensile force in the width direction of the glass ribbon and a traction force in the flow direction.
Such minute irregularity and undulation do not become a major problem in the case where the glass substrate is used as a glass sheet for automobiles, buildings, and the like but the minute irregularity and undulation become a cause of imparting distortion and color unevenness to images on a display to be fabricated in the case where it is used as a glass substrate for various displays. Therefore, particularly, in the case where a glass sheet formed by the float process is used as a glass substrate for a display, it is necessary to remove the minute irregularity and undulation by polishing the surface of the glass sheet.
On the surface of the glass sheet after forming, the minute irregularity and undulation remain. Among them, it is conventionally considered that undulation height at a 20 mm pitch imparts a large influence on polishing ability of the glass sheet and quality of a liquid crystalline display. For example, in Patent Document 1, for the purpose of improving the polishing ability, there is disclosed a method for manufacturing a glass substrate in which a float glass whose undulation height at a 20 mm pitch is 0.3 μm or less is selected.    Patent Document 1: JP-A-3-65529